Adaptive immune responses in jawed vertebrates are initiated when antigens are recognized by specific lymphocyte receptors. Antigen receptor diversity is generated via recombination of variable, diversity and joining gene segments in the immunoglobulin (Ig) and T cell receptor (TCR) gene loci. This combinatorial rearrangement generates vast repertoires of antibodies against unprocessed antigens and of TCRs that recognize antigen fragments presented within the cusp of major histocompatibility complex (MHC) class I and II molecules. Clonally diverse lymphocytes thus form the cornerstone of vertebrate adaptive immunity in the form of Ig bearing B cells and TCR bearing T cells that differentiate from stem cell precursors within primary hematopoietic tissues and the thymus. Cardinal elements of this recombinatorial immune system are conserved in all jawed vertebrates and the multigene TCR and Ig loci are remarkably complex even in the most basal gnathostome representatives, sharks, skates, and rays (Rast et al., 1997; Flajnik and Kasahara, 2001; Flajnik, 2002).
There is also abundant evidence for adaptive immunity in the jawless vertebrates, lamprey and hagfish, the only surviving descendents from the early vertebrate radiation (Forey and Janvier, 1993). Humoral and cell mediated types of immunologic responses have been reported for these agnathans. For example, lampreys produce specific circulating agglutinins in response to primary antigenic stimulation, make higher agglutinin levels after booster immunization (Finstad and Good, 1964; Marchalonis and Edelman, 1968; Litman et al., 1970; Pollara et al., 1970; Good et al., 1972; Hagen et al., 1985), reject second set skin allografts at an accelerated rate (Finstad et al., 1964; Perey et al., 1968; Good et al., 1972; Fujii and Hayakawa, 1983) and exhibit delayed type hypersensitivity reactions (Finstad and Good, 1964; Good et al., 1972). Agnathan adaptive immune responses have been attributed to cells that morphologically resemble the lymphocytes found in the lympho-hematopoietic tissues and blood of jawed vertebrates (Finstad and Good, 1964; Finstad et al., 1964; Perey et al., 1968; Cooper, 1971; Piavis and Hiatt, 1971; Good et al., 1972; Kilarski and Plytycz, 1981; Zapata et al., 1981; Fujii, 1982; Fujii and Hayakawa, 1983; Ardavin and Zapata, 1987; Mayer et al., 2002a). Like their mammalian counterparts, lamprey lymphocytes are more irradiation sensitive than other blood cell types (Good et al., 1972), aggregate and proliferate in response to antigenic stimulation (Finstad and Good, 1964; Cooper, 1971; Piavis and Hiatt, 1971), and express transcription factors that are involved in mammalian lymphocyte differentiation, such as PU.1/Spi-B and Ikaros (Haire et al., 2000; Shintani et al., 2000; Anderson et al., 2001; Mayer et al., 2002b). Surprisingly, however, Ig, TCR, and MHC genes have not been previously identified in jawless vertebrates or in the genome sequence of the invertebrate urochordate Ciona intestinalis (Azumi et al., 2003). The present invention relates to a novel lymphocyte receptor and nucleic acids that encode a novel lymphocyte receptor.